gcov - coverage testing tool at Linux.org

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GCOV(1) GNU GCOV(1)

NAME
gcov - coverage testing tool

DESCRIPTION
gcov is a test coverage program. Use it in concert with GCC to analyze your programs to help create more
efficient, faster running code and to discover untested parts of your program. You can use gcov as a
profiling tool to help discover where your optimization efforts will best affect your code. You can also use
gcov along with the other profiling tool, gprof, to assess which parts of your code use the greatest amount of
computing time.

Profiling tools help you analyze your code's performance. Using a profiler such as gcov or gprof, you can
find out some basic performance statistics, such as:

· how often each line of code executes

· what lines of code are actually executed

· how much computing time each section of code uses

Once you know these things about how your code works when compiled, you can look at each module to see which
modules should be optimized. gcov helps you determine where to work on optimization.

Software developers also use coverage testing in concert with testsuites, to make sure software is actually
good enough for a release. Testsuites can verify that a program works as expected; a coverage program tests
to see how much of the program is exercised by the testsuite. Developers can then determine what kinds of
test cases need to be added to the testsuites to create both better testing and a better final product.

You should compile your code without optimization if you plan to use gcov because the optimization, by
combining some lines of code into one function, may not give you as much information as you need to look for
`hot spots' where the code is using a great deal of computer time. Likewise, because gcov accumulates
statistics by line (at the lowest resolution), it works best with a programming style that places only one
statement on each line. If you use complicated macros that expand to loops or to other control structures,
the statistics are less helpful---they only report on the line where the macro call appears. If your complex
macros behave like functions, you can replace them with inline functions to solve this problem.

gcov creates a logfile called sourcefile.gcov which indicates how many times each line of a source file
sourcefile.c has executed. You can use these logfiles along with gprof to aid in fine-tuning the performance
of your programs. gprof gives timing information you can use along with the information you get from gcov.

gcov works only on code compiled with GCC. It is not compatible with any other profiling or test coverage
--all-blocks
Write individual execution counts for every basic block. Normally gcov outputs execution counts only for
the main blocks of a line. With this option you can determine if blocks within a single line are not
being executed.

-b
--branch-probabilities
Write branch frequencies to the output file, and write branch summary info to the standard output. This
option allows you to see how often each branch in your program was taken. Unconditional branches will not
be shown, unless the -u option is given.

-c
--branch-counts
Write branch frequencies as the number of branches taken, rather than the percentage of branches taken.

-n
--no-output
Do not create the gcov output file.

-l
--long-file-names
Create long file names for included source files. For example, if the header file x.h contains code, and
was included in the file a.c, then running gcov on the file a.c will produce an output file called
a.c##x.h.gcov instead of x.h.gcov. This can be useful if x.h is included in multiple source files and you
want to see the individual contributions. If you use the -p option, both the including and included file
names will be complete path names.

-p
--preserve-paths
Preserve complete path information in the names of generated .gcov files. Without this option, just the
filename component is used. With this option, all directories are used, with / characters translated to #
characters, . directory components removed and unremoveable .. components renamed to ^. This is useful
if sourcefiles are in several different directories.

-r
--relative-only
Only output information about source files with a relative pathname (after source prefix elision).
Absolute paths are usually system header files and coverage of any inline functions therein is normally
uninteresting.

-f
--function-summaries
Output summaries for each function in addition to the file level summary.

-o directory|file
--object-directory directory
--object-file file
Specify either the directory containing the gcov data files, or the object path name. The .gcno, and
.gcda data files are searched for using this option. If a directory is specified, the data files are in
that directory and named after the input file name, without its extension. If a file is specified here,
the data files are named after that file, without its extension.

-s directory
--source-prefix directory

Display the progress on the standard output.

gcov should be run with the current directory the same as that when you invoked the compiler. Otherwise it
will not be able to locate the source files. gcov produces files called mangledname.gcov in the current
directory. These contain the coverage information of the source file they correspond to. One .gcov file is
produced for each source (or header) file containing code, which was compiled to produce the data files. The
mangledname part of the output file name is usually simply the source file name, but can be something more
complicated if the -l or -p options are given. Refer to those options for details.

If you invoke gcov with multiple input files, the contributions from each input file are summed. Typically
you would invoke it with the same list of files as the final link of your executable.

The .gcov files contain the : separated fields along with program source code. The format is

<execution_count>:<line_number>:<source line text>

Additional block information may succeed each line, when requested by command line option. The
execution_count is - for lines containing no code. Unexecuted lines are marked ##### or ====, depending on
whether they are reachable by non-exceptional paths or only exceptional paths such as C++ exception handlers,
respectively.

Some lines of information at the start have line_number of zero. These preamble lines are of the form

-:0:<tag>:<value>

The ordering and number of these preamble lines will be augmented as gcov development progresses --- do not
rely on them remaining unchanged. Use tag to locate a particular preamble line.

The additional block information is of the form

The information is human readable, but designed to be simple enough for machine parsing too.

When printing percentages, 0% and 100% are only printed when the values are exactly 0% and 100% respectively.
Other values which would conventionally be rounded to 0% or 100% are instead printed as the nearest non-
boundary value.

When using gcov, you must first compile your program with two special GCC options: -fprofile-arcs
-ftest-coverage. This tells the compiler to generate additional information needed by gcov (basically a flow
graph of the program) and also includes additional code in the object files for generating the extra profiling
information needed by gcov. These additional files are placed in the directory where the object file is
located.

Running the program will cause profile output to be generated. For each source file compiled with
-fprofile-arcs, an accompanying .gcda file will be placed in the object file directory.

Running gcov with your program's source file names as arguments will now produce a listing of the code along
with frequency of execution for each line. For example, if your program is called tmp.c, this is what you see
when you use the basic gcov facility:

$ gcc -fprofile-arcs -ftest-coverage tmp.c
$ a.out
$ gcov tmp.c

-: 3:int main (void)
1: 4:{
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
10: 10: total += i;
-: 11:
1: 12: if (total != 45)
#####: 13: printf ("Failure\n");
-: 14: else
1: 15: printf ("Success\n");
1: 16: return 0;
-: 17:}

When you use the -a option, you will get individual block counts, and the output looks like this:

-: 0:Source:tmp.c
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
1: 4:{
1: 4-block 0
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
11: 9-block 0
10: 10: total += i;
10: 10-block 0
-: 11:
1: 12: if (total != 45)
1: 12-block 0
#####: 13: printf ("Failure\n");
$$$$$: 13-block 0
-: 14: else
1: 15: printf ("Success\n");
1: 15-block 0
1: 16: return 0;
1: 16-block 0
-: 17:}

In this mode, each basic block is only shown on one line -- the last line of the block. A multi-line block
will only contribute to the execution count of that last line, and other lines will not be shown to contain
code, unless previous blocks end on those lines. The total execution count of a line is shown and subsequent
lines show the execution counts for individual blocks that end on that line. After each block, the branch and
call counts of the block will be shown, if the -b option is given.

Here is a sample of a resulting tmp.c.gcov file:

-: 0:Source:tmp.c
-: 0:Graph:tmp.gcno
-: 0:Data:tmp.gcda
-: 0:Runs:1
-: 0:Programs:1
-: 1:#include <stdio.h>
-: 2:
-: 3:int main (void)
function main called 1 returned 1 blocks executed 75%
1: 4:{
1: 5: int i, total;
-: 6:
1: 7: total = 0;
-: 8:
11: 9: for (i = 0; i < 10; i++)
branch 0 taken 91% (fallthrough)
branch 1 taken 9%
10: 10: total += i;
-: 11:
1: 12: if (total != 45)
branch 0 taken 0% (fallthrough)
branch 1 taken 100%
#####: 13: printf ("Failure\n");
call 0 never executed
-: 14: else
1: 15: printf ("Success\n");
call 0 called 1 returned 100%
1: 16: return 0;
-: 17:}

For each function, a line is printed showing how many times the function is called, how many times it returns
and what percentage of the function's blocks were executed.

For each basic block, a line is printed after the last line of the basic block describing the branch or call
that ends the basic block. There can be multiple branches and calls listed for a single source line if there
are multiple basic blocks that end on that line. In this case, the branches and calls are each given a
number. There is no simple way to map these branches and calls back to source constructs. In general,
though, the lowest numbered branch or call will correspond to the leftmost construct on the source line.

For a branch, if it was executed at least once, then a percentage indicating the number of times the branch
was taken divided by the number of times the branch was executed will be printed. Otherwise, the message
"never executed" is printed.

For a call, if it was executed at least once, then a percentage indicating the number of times the call
returned divided by the number of times the call was executed will be printed. This will usually be 100%, but
may be less for functions that call "exit" or "longjmp", and thus may not return every time they are called.

The execution counts are cumulative. If the example program were executed again without removing the .gcda
file, the count for the number of times each line in the source was executed would be added to the results of
the previous run(s). This is potentially useful in several ways. For example, it could be used to accumulate
data over a number of program runs as part of a test verification suite, or to provide more accurate long-term
information over a large number of program runs.

if (a != b)
c = 1;
else
c = 0;

can be compiled into one instruction on some machines. In this case, there is no way for gcov to calculate
separate execution counts for each line because there isn't separate code for each line. Hence the gcov
output looks like this if you compiled the program with optimization:

100: 12:if (a != b)
100: 13: c = 1;
100: 14:else
100: 15: c = 0;

The output shows that this block of code, combined by optimization, executed 100 times. In one sense this
result is correct, because there was only one instruction representing all four of these lines. However, the
output does not indicate how many times the result was 0 and how many times the result was 1.

Inlineable functions can create unexpected line counts. Line counts are shown for the source code of the
inlineable function, but what is shown depends on where the function is inlined, or if it is not inlined at
all.

If the function is not inlined, the compiler must emit an out of line copy of the function, in any object file
that needs it. If fileA.o and fileB.o both contain out of line bodies of a particular inlineable function,
they will also both contain coverage counts for that function. When fileA.o and fileB.o are linked together,
the linker will, on many systems, select one of those out of line bodies for all calls to that function, and
remove or ignore the other. Unfortunately, it will not remove the coverage counters for the unused function
body. Hence when instrumented, all but one use of that function will show zero counts.

If the function is inlined in several places, the block structure in each location might not be the same. For
instance, a condition might now be calculable at compile time in some instances. Because the coverage of all
the uses of the inline function will be shown for the same source lines, the line counts themselves might seem
inconsistent.

Long-running applications can use the "_gcov_reset" and "_gcov_dump" facilities to restrict profile collection
to the program region of interest. Calling "_gcov_reset(void)" will clear all profile counters to zero, and
calling "_gcov_dump(void)" will cause the profile information collected at that point to be dumped to .gcda
output files.

SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), gcc(1) and the Info entry for gcc.

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free
Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with the
Invariant Sections being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being
(a) (see below), and with the Back-Cover Texts being (b) (see below). A copy of the license is included in
the gfdl(7) man page.

(a) The FSF's Front-Cover Text is: